Just in front of the AF sensor we find the SIR optics, sometimes also called separator lenses. The image is
from the Canon 5D. The image below is also from the 5D and shows the AF sensor. Comparing the two images
it is easy to imagine how each lens serves its own area of AF pixel arrays (the black horisontal and vertical bars)


The pattern of black AF pixel arrays above do however not directly match the AF point layout seen
in the view finder at first glance. The secret lies in the fact that each AF point needs two arrays to work.
The AF point is placed at the center of the distance between the midpoint of each array in a pair.
In the bottom part of the figure below each AF point is numbered. For each number and hence AF point,
there is one or more AF array pairs (a and b) in the upper figgure part. The blue bars correspond
to the f/5.6 sensitive arrays and the red bars to f/2.8 sensitive arrays. A vertical array is sensitive
to contrast in the vertical direction, e.g. from a horizontal edge. Likewise a horizontal array is sensitive
to contrast in the horizontal direction, e.g. from a vertical edge such as a door frame. Note that the
shape of the AF point in the viewfinder indicates the direction of sensitivity.    




With the knowledege gained above it's now possible to overlay the superposition of the a and b array
parts onto the corresponding viewfinder AF points as seen below